Part 1: Alcoholic fermentations
(Readers please note: This is the first in a series of wine-related topics that have been selected in response to many questions from home winemakers.)
Home Winemaker’s question or inquiry: “From time to time I have had a wine grape / juice fermentation that works either very slowly or after starting, slows to an eventual halt. I often have trouble re-starting the fermentation and would like to know what may be causing this and what, if anything, I can do about it.”
Discussion: Relax ! You’ve got lots of company when it comes to this topic, as this is the most often asked question we receive throughout the year. Presented here is information as well as suggestions that other home winemakers have found helpful.
Topic I : Stuck and Sluggish Fermentations 1
A) Stuck Fermentations: Diagnosis
The most common causes of stuck or sluggish fermentations include:
- Nutrient Deficiency
- Temperature Extremes
- Microbial Incompatibility
- Toxic Substances
- Deficient Yeast Strain
Most contemporary winemakers recognize the fact that one of the most commonly occurring problems during the winemaking process that can cause stuck or sluggish fermentations is a nutrient deficiency.
One can make the argument that nutrient deficiency can be diagnosed using laboratory analysis. While this is true, many winemakers do not conduct a proper nutrient analysis at the time of initiating the fermentation, most often due to costs, time constraints, insufficient knowledge on the subject or lack of equipment required for the tests.
That said, it is current practice in most California wineries for example, to treat the fermentation as if it is nutrient deficient (or will be deficient at some point) and add yeast nutrients such as diammonium phosphate (DAP) at the start. Other wineries in the U.S. often add other proprietary yeast nutrients at this time.
Also, it should be noted that the timing of the addition(s) is often critical that the yeast population conducting the fermentation gains maximum benefit from the addition. Feeding too early can result in feeding the wrong population of microorganisms. Late feedings may not be beneficial either, due to the fact that high ethanol concentrations can cause an arrest of the uptake of nitrogen compounds required for the completion of the fermentation.
Many winemakers believe that the addition of yeast nutrients is best done after approximately 1/3 of the fermentation has been completed to help ensure that the yeast gain maximum benefit.
Another common cause of a stuck or sluggish fermentation is exposing the fermentation either to a temperature extreme or sudden temperature shock to the yeast population.
This problem is more common in red wine production because red wines are often fermented at higher temperatures (85⁰F or higher) and the maximum temperature that most yeast can tolerate is between 95 and 105⁰C, beyond which the yeast start to die or become stressed, resulting in the production of noxious odors. In addition, winemakers point out that larger fermentations can give off significant amounts of heat which can cause serious problems for the winemaker and that high temperature(s) can not only inhibit fermentation but also render the yeast population more susceptible to arrest at high alcohol levels (around 11%). If the culture arrests at this point and if sugar is still present in significant quantities, the stage may be set for spoilage organisms to invade the fermentation and cause additional headaches for the winemaker.
Toxic substances may sometimes present themselves during the fermentation, often arising from various sources such as fungi, and certain molds or bacteria. Many of these substances can be inhibitory to yeast depending on the composition of the must or juice.
Another problem encountered by winemakers is that their fermentation often seems to arrest following any microbial “bloom”. (A diagnosis of this potential problem is usually done by visual inspection, as well as what is called the ‘sniff test’. The explanation here is that stressed yeast, as it has been pointed out earlier, will often give off noxious smells and “stinky” aromas that are easily detected by smelling a small sample of the fermentation) In these cases it may possibly be due to competition for nutrients or production by the offending bacteria of a toxic substance such as acetic acid. Interestingly, nutrient supplementation does not appear to solve this problem, and if it is, in fact, competition for some specific nutrient, it may be that the particular nutrient is something other than what is typically found in popular nutritional supplements.
Home winemakers often note that certain vineyards or grape growing regions tend to cause problem fermentations more frequently than similar grapes harvested from other vineyards in a geographically similar area.
While different theories to explain this have been put forth, a likely explanation is the presence of inhibitory compounds that are commonly found in varying amounts in, of all places, the grape seeds.
Additionally, one problem often ignored is the presence of residual sprays that are used to control disease or insects. These substances can have potentially serious effects on the quality of the grapes going into the fermentation, often resulting in poor performance by the yeast. Some home winemakers have suggested washing the grapes prior to crushing, thus providing a reduction in the amount of residual spray going into the fermentation. We have evaluated this additional step with mixed results. The additional labor and the time involved do not seem to be worth the effort.
Some widely used commercial yeast strains have been shown to be more sensitive to various stresses and this suggests that it can be advantageous to match specific yeast strains to different fermentation conditions. In any of these potentially serious situations, the advice / assistance of a trained winemaker or enologist are recommended.
B) Stuck Fermentations: Treatment
Scientists point out that stuck fermentations can be difficult to treat for a number of reasons:
- Yeast cells can adapt to adverse fermentation conditions by reducing fermentation capacity.
- Once biological adaptation has occurred, it can be difficult to reverse since these adaptations are a survival mechanism.
- Additionally, if the exact cause of the arrest of fermentation is not known, it can be difficult, or perhaps impossible to determine the cause of the stress for yeast cells.
- Conditions of stress leading to a resting state frequently are lethal or near lethal situations for the yeast.
- If uncorrected, a significant percentage of the population may die and release compounds into the fermentation that signal that conditions now exist that will lead to cell death if not corrected immediately. It’s important to remember that many microbes are social organisms and respond to signals generated by other members of the same species in the environment. For example, massive cell death appears to lead to the release of these compounds, signaling other members in the colony of the same species to reduce activity or “shut down” until environmental conditions are improved significantly.
If an attempt to re-start the fermentation using a new inoculum is performed incorrectly, it is hypothesized that the new cells placed in the same environment will often respond to these “turn off the lights, the party’s over” signals, leading to an arrest of these additional cells in the new inoculum. Stated another way, we’re still at the starting gate.
Re-Initiation of Stuck Fermentations
General guideline for fermentation re-initiation:
- Careful identification of the nature/origin of the problem
- -If re-inoculating, making sure that the inoculum is adapted to conditions of stuck fermentation
- -Use of serial-inoculum procedure
- -possible removal of existing biomass
As much as is possible, it will be necessary to ensure that the new inoculum is adapted to the conditions of the stuck ferment such as pH, temperature and ethanol content. This is done to prevent shock to the inoculum upon addition to the ferment and is usually done using a procedure called serial re-inoculation. Here, a modest amount of the stuck wine/juice is slowly mixed with an equal volume of new juice or actively fermenting must in as close to conditions in the stuck/sluggish fermentation as possible.
Start by using a clean bowl or other clean container of a size to comfortably fit all the fermentation ingredients, both new and stuck. Prepare a known amount of warm water (say 1 pint under 100 degrees F) mixed with 1 level teaspoon of table sugar and a pinch of yeast nutrient. To this add two 5 gram packages of fresh yeast culture and let stand about 20 minutes. At this point you should see evidence of the yeast starting to work. Now take 1 pint of the stuck culture and add slowly a small amount of this to the fresh, fermenting culture. This mixture is allowed to ferment for about 30 minutes and at that time if everything appears OK, slowly add another equally small portion of the stuck culture to the new fermentation. If everything continues to go well, continue these steps until both ferments are working together. The next step is to slowly re-introduce the new combined fermenting culture into the bulk portion of the stuck fermentation by slowly ladling the new over the top of the stuck fermentation without stirring. This is then left to restart over the next 12-24 hours, at which time the re-started fermentation should be well underway.
- Adapted from Bisson,L.F. and C.E. Butzke. 2000. Diagnosis and Rectification of Stuck and Sluggish Fermentations. Am. J. Enol. Vitc. 51(2):168-177.